1. Field of the Invention
This invention relates to a tire building apparatus for use in forming a green tire from a green case.
2. Related Art Statement
Tire building apparatuses have been known, for example, as disclosed in Japanese Patent Application Publication No. 55-1181. This disclosed tire building apparatus includes a main hollow shaft, and a rotating screw shaft inserted in the main shaft and formed on both axial end portions with screw threads in opposite directions. The apparatus further includes a pair of first sliders axially movably supported on the outer circumference of the main shaft at both axial end portions and connected to the screw shaft to move through equal distances toward and away from each other upon rotating the screw shaft, and a pair of second sliders axially movably supported on the outer circumference of the first sliders. The apparatus further includes a pair of support members expansibly and contractibly supported by the second sliders and having engagement portions adapted to engage inner surfaces of bead portions of a green case so that the bead portions are supported on their insides by the support members whose diameters increase, and a pair of cylinders for axially moving the second sliders together with the support members. Moreover, the apparatus includes a drum core substantially in the form of a ring composed of a plurality of segments circumferentially spaced and provided between the first sliders to surround the main shaft, and a link mechanism composed of a plurality of links connecting the drum core and the first sliders for expanding the drum core to increase its diameter when the first sliders move toward each other and contracting the drum core to reduce its diameter when the first sliders move away from each other.
In order to produce green tires of high quality, it is required to bring the width center portion of the drum core into coincidence with the axial center portion of a cylindrical green case. For the purpose of accomplishing such a coincidence of the center portions with the disclosed tire building apparatus, after the engagement portions of the support members have been engaged with inner surfaces of bead portions of a green case, the second sliders are moved axially outwardly together with the support members by means of the cylinders to pull the bead portions of the green case axially outwardly.
In the apparatus above described, the pair of cylinders are used for moving the support members. However, it is difficult to obtain two cylinders whose strokes in pulling operation are completely equal to each other. Therefore, this apparatus encounters a difficulty that the green case can be axially positioned only with low accuracy. Moreover, provision of these cylinders makes complicated the construction resulting in a large-sized apparatus.
In order to solve these problems, it may be conceivable that such cylinders are dispensed with, and even after the drum core has been contracted to reduce its diameter, the screw shaft is continuously rotated so as to axially outwardly move the first and second sliders to pull the bead portions of a green case axially outwardly. With such an arrangement, however, the radial movement of the drum core becomes large because the drum core further radially contracts to a smaller diameter than the reduced diameter. Such a large radial movement of the drum core could not be realized because it may exceed the limit stroke of the link mechanism. If it is intended to increase the limit stroke of the link mechanism, thicknesses and widths of the links unavoidably become too small so that the strength of the links would lower exceeding a range of practical use.